EASTERN MASSACIIUvSETTS. 



423 



"The porpliyrios appear to over] io tlu; Salem syenites uncoufonuaLly, ami 



together willi litem are cut l)y at least two series of dioritii' dykes The 



porphyries, though varying greatly in aspect and in composiUon, are neverthe- 

 less but ene I\)rnial:.ion, and derived from a vast conglomerate Avhich iippearn in 

 Lj^nn, Saugus, and Marbleliead, and is reported to occur under the granites on 

 the Beverly shore. The t)riginally conglomerate nature ol" the entire (h'[)osit is 

 infurred by extensive observations made by myself at Marblehead Xeck, and 

 l)y my assistant,' Mr. W. 0. Crosby, in Saugus, and the general identity of tJie 

 purely crystalline porpliyrles of Lynn with tliose of Marhlehead Keck, which 



are unduubledly merely altered conglomerates The change into the 



felsitc is the most instructive, since here it is possible to trace the included 

 pebbk; of dark colored, banded porphyry through all stages until it hec(nnes a 

 mere s[)ot In the ligbt colored matrix. Durin.g this change the pebble disap- 

 pears by some ])rocess by which the structure is altered from without, the 



centre bcim; the last point to lose its distinctive colorim^ or structure 



The fact seems to me un(|iiestionable, .... that both a felsite and a true [)or- 

 phyry were formed out of a conglomerate, without any perceptil.de clainge 

 having been made in the form of the contained pebbles." (Proc. l>ost. Soc. 

 Nat. Hist., lS75^7f), XYIIL, pp. 220-22:).) 



rrofowsor Tljatt points out that the felsite pebbles iu the conglomor- 

 ato are dilferout from the felaito formed from it, and holds that through 

 prcHsuro or otherwise the pebbles form the thin laniiuiu seen in the 

 banded felsite. The reader is refcri'ed to his very interesting descrip- 

 tion of the imaginary pi'ocesses, siueo it is too lengthy to transcribe 

 here. (Ibid., pp. 22.3, 224.) 



Mr. W. 0. Crosby, in his Jlcport on the Geological Map of JMassa- 

 chusettSj 1870, says (pp. 7, 8, 10, 11) : 



" The Ko/oic rocks of Massachusetts may to a Lirgo extentat least he di\ ided 

 really and chronologically, into three divisions, whicli, stated in Iheir 

 order of seipience, are the Norian, the Iluronian, and the Mont Alban. I 

 weiu"h mv words well when I describe these divisions as both litholon-ical and 

 chronological; for, .... I do not hcaltate to affmn that the lithological char- 

 acters of the divisions which have been worked out amoue the crvstalllnes of 

 this region, — the chromdogical and geographical distinctness of whicli I can- 

 not doubt — are as unlike as the faiuix of any two successive geological for- 

 mations." 



This statement was in substance again affirmed iu 1880. Of the 

 Norian he wiates ;^ 



litlii'iog 



"The rocks of this formation, though fretpieutly stratified, seem in general to 

 have been st^mewhat fluent, and usually exhibit more gr less extravasation; 

 but dmdtiless in some cases the metamorphii.- action has stopped short of this 

 extreme term, though destroying all traces of bedding. In many places . . . . 



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